This invention relates to methods for preparing certain substituted cyclopentane compounds and certain intermediates thereof. The present invention is also concerned with novel intermediates or precursors for producing the substituted cyclopentane compounds. Substituted cyclopentane compounds prepared according to the present invention are useful as neuraminidase inhibitors, and especially in pharmaceutical composition for preventing, treating or ameliorating viral, bacterial and other infections.
Despite the wealth of information available, influenza remains a potentially devastating disease of man, lower mammals, and birds. No effective vaccine exists and no cure is available once the infection has been initiated.
Influenza viruses consist of eight pieces of single stranded RNA, packaged in orderly fashion within the virion. Each piece codes for one of the major viral proteins. The replication complex is enclosed with a membrane composed of matrix protein associated with a lipid bilayer. Embedded in the lipid bilayer are two surface glycoprotein spikes, hemagglutinin (HA) and the enzyme neuraminidase (NA). All of the viral genes have been cloned and the three-dimensional structures of the surface glycoproteins have been determined.
Influenza viruses continually undergo antigenic variation in the two surface antigens, HA and NA, toward which neutralizing antibodies are directed. For this reason, vaccines and a subject""s natural immune system have not been very effective. Attention is now being directed to finding other potential antiviral agents acting at other sites of the virion.
Furthermore, many other organisms carry NA. Many of these NA-possessing organisms are also major pathogens of man and/or mammals, including Vibraeo cholerae, Clostridium perfringes, Streptococcus pneumonia, Arthrobacter sialophilas, and other viruses, such as parainfluenza virus, mumps virus, Newcastle disease virus, fowl plague virus, and Sendai virus. Compounds of this invention are also directed to inhibiting NA of these organisms.
In viruses, NA exists as a tetramer made of four roughly spherical subunits and a centrally-attached stalk containing a hydrophobic region by which it is embedded in the organism""s membrane. Several roles have been suggested for NA. The enzyme catalyzes cleavage of the xcex1-ketosidic linkage between terminal sialic acid and an adjacent sugar residue. Removal of the sialic acid lowers the viscosity and permits access of the virus to the epithelial cells. NA also destroys the HA receptor on the host cell, thus allowing elution of progeny virus particles from infected cells.
Research indicates that the active site for influenza neuraminidase remains substantially unchanged for the major strains of influenza. For example, a comparison of sequences from influenza A subtypes and influenza B shows conserved residues with crucial structural and functional roles. Even though the sequence homology is only about 30%, many of the catalytic residues are conserved. Furthermore, the three-dimensional structures of influenza A and B neuraminidases have been determined. Superposition of the various structures shows remarkable structural similarity of the active site. Since the active site amino acid residues are conserved in all known influenza A neuraminidases that have been sequenced so far, an inhibitor that is effective against different strains of influenza A and/or B neuraminidase can be designed based on the three-dimensional structure of a neuraminidase.
In general, the role of NA is thought to be for the mobility of the virus both to and from the site of infections. Compounds that inhibit neuraminidase""s activity may protect a subject from infection and/or cure a subject once infection has set in.
Analogs of neuraminic acid, such as 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (DANA) and its derivatives are known to inhibit HA in vitro; however, these compounds are inactive in vivo. Palese and Schulman, in CHEMOPROPHYLAXIS AND VIRUS INFECTION OF THE UPPER RESPIRATORY TRACT, Vol. 1 (J. S. Oxford, Ed.), CRC Press, 1977, at PS 189-205.
Von Itzstein et al. describes cyclohexane analogs of xcex1-D-neuraminic acid of the formula 
wherein:
A is O, C or S in Formula (a), and N or C in Formula (b); R1 is CO2H, PO3H2, NO2, SO2H, SO3H, tetrazolyl-, CH2CHO, CHO, or CH(CHO)2;
R2 is H, OR6, F, Cl, Br, CN, NHR6, SR6 or CH2X, where X is NHR6 halogen, or OR6;
R3 and R3xe2x80x2 are H, CN, NHR6, SR6, xe2x95x90NOR6, OR6, guanidino, NR6;
R4 is NHR6, SR6, OR6, CO2R6, NO2, C(R6)3., CH2CO2R6, CH2NO2 or CH2NHR6;
R5 is CH2YR6, CHYR6CH2YR6 or CHYR6CHYR6CH2YR6;
R6 is H, acyl, alkyl, allyl, or aryl;
Y is O, S, NH, or H;
and pharmaceutical salts thereof, useful as antiviral agents.
In addition, certain benzene derivatives are suggested in U.S. Pat. No. 5,453,533 as being inhibitors of influenza virus neuraminidase and various others are disclosed in U.S. patent application Ser. No. 08/413,886. Yamamoto et al. describe various sialic acid isomers as having inhibitory activity against neuraminidase in Synthesis of Sialic Acid Isomers With Inhibitory Activity Against Neuraminidase, TETRAHEDRON LETTERS, Vol. 33, No. 39, pp. 5791-5794, 1992.
WO 96/26933 to Gilead Sciences, Inc. describes certain 6-membered ring compounds as possible inhibitors of neuraminidase.
More recently, there have been disclosed new cyclopentane derivatives that are useful as neuraminidase inhibitors. For example, see WO 96/30329, assigned to BioCryst Pharmaceuticals, Inc., the assignee of the present application, the entire disclosure of which being incorporated herein by reference.
The present invention relates to methods for preparing certain substituted cyclopentane compounds that are useful as inhibitors of the enzyme neuraminidase. Moreover, the present invention is concerned with a method for preparing certain precursors of the substituted cyclopentane compounds.
The substituted cyclopentane compounds prepared according to the present invention are represented by the following formulae 1a and 1b: 
wherein each R1 individually is alkyl or substituted alkyl, alkenyl or substituted alkenyl of 1-6 carbon atoms, or H; each of R2 and R3 individually is alkyl or alkenyl of 1-8 carbon atoms, cycloalkyl or substituted cycloalkyl of 4-8 carbon atoms, aryl or substituted aryl, arylalkyl or substituted arylalkyl, or H provided at least one of R2 and R3 is other than H; X is NHR1, NHC (xe2x95x90NH) NHR4 where R4 is H, alkyl of 1-6 carbon atoms, OR1, COR1, COOR1, CN or NO2; A is H, F, OR1, OCOR1, xe2x80x94OOCNHR1, NHR1, or NHCOOR1; and
pharmaceutically acceptable salts thereof.
The precursors according to the present invention are isoxazoline derivatives represented by the following formula 4: 
wherein R2 and R3 are the same as defined above and wherein each of Y and Z individually is COOR1 or H provided that at least one of Y and Z is other than H.
The isoxazoline derivatives according to formula 4 are prepared according to the following procedure:
A nitrile oxide of the formula 2 
is reacted with a cyclopentene derivative of the formula 3 to produce the desired isoxazoline derivative. R2, R3, Y and Z are the same as defined above.
The cyclopentane compounds of formula 1a can be prepared from the above isoxazoline derivatives by reducing the isoxazoline derivatives of formula 4 to form an aminoalcohol derivative according to formula 5. Reacting the aminoalcohol compound of formula 5 with an anhydride or acid halide of a carboxylic acid of the formula: R1COOH to produce the acylated compounds represented by formula 6. Next, the alcohol group of the acylated compounds is converted into a leaving group which in turn is displaced by ammonia or guanidine to produce compounds of formula 1a or the leaving group is displaced by an azide ion which in turn is converted to the guanidine using NH2 compound.
In an alternative process for preparing the cyclopentane compounds of formula 1a, an isoxazoline compound of formula 4 is converted to ketone according to formula 7 
by opening its isoxazoline ring. The ketone of formula 7 is subjected to reductive amination to form a compound according to formula 8 
wherein R4 is H or a substituted benzyl. When R4 is a substituted benzyl, such is removed to give the aminoalcohol compounds of formula 5. The aminoalcohols are then converted to the final product as discussed above.
According to a still further aspect of the present invention, cyclopentane derivatives of formula 9 can be reacted with a nitrile oxide of formula 2 to give the isoxazoline derivatives 10 as shown in Scheme 2. Such isoxazolines may be converted to compounds 12 and may further be dehydrated to give the unsaturated compounds 13. Alternatively, the OH may be converted to NH2 or F by conventional methods known in the art to afford compounds 14 and 15 respectively.
It is a further object of this invention to provide a method of using compounds of this invention for treating and/or curing a viral infection.